1. Field of the Invention
The present invention relates to a force sensor and a method for producing the force sensor, and particularly to a force sensor in which a force sensor chip and an attenuator are joined by anodic bonding through a glass layer sandwiched therebetween, and a method for producing the force sensor.
2. Description of the Related Art
Conventionally, in an industrial robot and the like, there has been adopted a multi-axis force sensor for accurately measuring a size and direction of an external force applied to the robot during an action of the robot, in order to implement a control under which the robot appropriately and flexibly responds to the external force.
As the multi-axis force sensor, for example, there has been known a force sensor utilizing a property of a strain resistive element (piezo resistive element), in which a resistance value changes in accordance with minute strain (compression, tension) caused by an applied external force (See, for example, Japanese unexamined patent publication Nos. 2003-207405 and 2003-254843, the disclosures of which are herein incorporated by reference in their entireties).
The multi-axis force sensor has a force sensor chip formed on a semiconductor substrate by a semiconductor production process, and an attenuator made of a metal member for accommodating and securing the force sensor chip.
Specifically, in the force sensor chip, the strain resistive elements are appropriately arranged around an action portion to which an applied external force is transmitted, and a change in a resistance value of the strain resistive element due to the external force is detected as an electrical signal, which presents a size and direction of the external force. If the applied external force is directly transmitted to the strain resistive element, and the external force is excessively large, the force sensor chip may be damaged. In order to receive the external force of various magnitudes without causing damage, the attenuator is introduced for attenuating the applied external force to an appropriate magnitude to transmit to the force sensor chip.
The force sensor chip is fixed to the attenuator through the joint portion, in order to accurately transmit the applied external force to the force sensor chip. If the force sensor chip formed on the semiconductor substrate is directly joined to the attenuator made of a metal member, there arise problems, such as electric hazard including leakage, detachment of two members at the joint portion due to a difference in coefficient of thermal expansion between two members, and thermal strain, all of which may deteriorate the detection accuracy.
In order to overcome the problems, some conventional techniques introduce a bulky (massive) glass plate, which has approximately the same thickness as that of the semiconductor substrate, as an interface between the force sensor chip and the attenuator, from a viewpoint of insulation property and coefficient of thermal expansion. For joining the glass plate to the force sensor chip and to the attenuator, an epoxy resin adhesive is applied to the joint faces thereof, or the joint faces are chemically bonded by anodic bonding. In anodic bonding, while the entire subject is heated, a voltage is applied to the subject with a negative voltage on a glass plate side and a positive voltage on a side of an object to be joined, in order to transfer alkali ion, such as Na+, from the glass to the object. Typical thickness of the glass plate to be joined to the object is approximately 0.1 to several mm.
The conventional anodic bonding method will be specifically described with reference to FIGS. 13A and 13B. FIGS. 13A and 13B illustrate steps of anodic bonding at joint portions of a force sensor chip and an attenuator with the presence of a glass plate therebetween, in which FIG. 13A shows joining of the glass plate and the attenuator, and FIG. 13B shows joining of the attenuator with the glass plate joined thereto and the force sensor chip.
In the anodic bonding, as described above, a voltage is applied to the subject with a negative voltage on the glass plate, and a positive voltage on an object to be joined.
Accordingly, when the attenuator and the glass plate are joined first by anodic bonding, as shown in FIG. 13A, a voltage is applied to the subject with a negative voltage on the glass plate 100 and a positive voltage on the attenuator 300. Then, when a complex of the attenuator 300 with the glass plate 100 joined thereto and the force sensor chip 200 are joined by anodic bonding, as shown in FIG. 13B, a voltage is applied with a negative voltage on an attenuator 300 side and thus on a glass plate 100 side of the joint portion 120, and a positive voltage on the force sensor chip 200.
It should be noted that, in this explanation of the conventional anodic bonding, the attenuator 300 and the glass plate 100 are joined first, and then the glass plate 100 and the force sensor chip 200 are joined; however, there may be a case in which the force sensor chip 200 and the glass plate 100 are joined first and then the glass plate 100 and the attenuator 300 are joined.
However, when an epoxy resin adhesive is used at joint faces of the glass plate and the force sensor chip or the attenuator, adhesion may become poor due to aged deterioration of the adhesive. In addition, a joint portion between the force sensor chip and the glass plate may be deformed or adhesive strength may become smaller, due to repeated compression and tension caused by external force on the attenuator. Due to this deterioration of the adhesive and deformation of joint portion, there arises a problem that minute change in external force cannot be accurately transmitted.
Further in anodic bonding, directions of voltage applied to the joint portion 110 are reversed between a case of joining of the attenuator 300 with the glass plate 100 (FIG. 13A), and a case of joining of the glass plate 100 with the force sensor chip 200 (FIG. 13B). As a result, fracture in the joint interface may occur from alkali ion (e.g., Na+) in the glass plate 100, leading to problems, such as reduced joint strength and detachment of the joint face.
These problems also occur in a case where the force sensor chip 200 is joined with the glass plate 100 first and then the glass plate 100 is joined with the attenuator 300.
In the conventional anodic bonding, a bulky glass plate having a thickness that is about the same as that of the force sensor chip is used. Therefore, deflection of the glass plate caused by an external force cannot be neglected. Because the deflection of the glass plate should be taken into account, design flexibility of the force sensor tends to be limited.
Further, when a machine processing, such as carving, is performed on the bulky glass plate in order to make a specific shape, microcracks (minute cracks that cannot be observed) will be generated. Since microcracks reduce a load-carrying capacity of the glass plate, there remains a problem of poor endurance and poor reliability.
Therefore, first, it would be desirable to provide a force sensor in which the joint portion is prevented from deteriorating during anodic bonding of a force sensor chip and an attenuator with a glass layer sandwiched therebetween, and a method for producing the same.
Second, it would be desirable to provide a force sensor in which the above-mentioned defects can be suppressed which may otherwise be generated by anodic bonding using a bulky glass plate, and a method for producing the same.